There is large and growing interest in making antimicrobial a wide variety of materials and surfaces. Textiles and other materials present in a hospital setting have been shown to be sufficient bacterial supports, raising the possibility that these materials could be responsible for disease transfer among hospital populations. A. N. Neely and M. P. Maley, J. Clin. Microbiol., 2000, 38 724; M. M. MacKinnon and K. D. Allen, J. Hosp. Infect., 2000, 46 216; and L. Scudeller, O. Leoncini, S. Boni, A. Navarra, A. Rezzani, S. Verdirosi, and R. Maserati, J. Hosp. Infect., 2000, 46 222. Thus, it may be possible to reduce infection rates by adding antimicrobial agents to textiles and other surfaces. There has been and continues to be a considerable amount of research into making fabrics antimicrobial to address this and other issues. U. Klueh, V. Wagner, S. Kelly, A. Johnson, and J. D. Bryers, J. Biomed. Mater. Res., 2000, 53 621; Y. Sun and G. Sun, Journal of Applied Polymer Science, 2003, 88 1032; S. Borman, Chemical and Engineering News, 2002, 80 36; J. Lin, S. Qiu, K. Lewis, and A. M. Klibanov, Biotechnology and Bioengineering, 2002, 83 168; S. M. A. El Ola, R. Kotek, W. C. White, J. A. Reeve, P. Hauser, and J. H. Kim, Polymer, 2004, 45 3215; T. Abel, J. I. Cohen, R. Engel, M. Filshtinskaya, A. Melkonian, and K. Melkonian, Carbohydrate Research, 2002, 337 2495; G. Sun and S. D. Worley, J. Chem. Educ., 2005, 82 60; T. Yuranova, A. G. Rincon, A. Bozzi, S. Parra, C. Pulgarin, P. Albers, and J. Kiwi, J. Photochem. Photobiol. A-Chem., 2003, 161 27; T. Tashiro, Macromolecular Materials and Engineering, 2001, 286 63; and H. L. Schreuder-Gibson, Q. Truong, J. E. Walker, J. R. Owens, J. D. Wander, and W. E. Jones, MRS Bulletin, 2003, 28 574. For instance, self-sterilizing fabrics are under study for biowarfare protection. In addition to fabrics, antimicrobial surfaces are of interest for medical devices to combat the insidious problem of biofilm formation, and for reduction of biofouling in water handling systems. I. Raad, et al., Ann. Intern. Med., 1997, 127 267; D. G. Maki, S. M. Stolz, S. Wheeler, and L. A. Mermel, Ann. Intern. Med., 1997, 127 257; H. Gollwitzer, K. Ibrahim, H. Meyer, W. Mittelmeier, R. Busch, and A. Stemberger, J. Antimicrob. Chemother., 2003, 51 585; E. L. Munson, S. O. Heard, and G. V. Doem, Chest, 2004, 126 1628; M. E. Rupp, T. Fitzgerald, N. Marion, V. Helget, S. Puumala, J. R. Anderson, and P. D. Fey, Am. J. Infect. Control, 2004, 32 445; I. C. Yue, J. Poff, M. E. Cortes, R. D. Sinisterra, C. B. Faris, P. Hildgen, R. Langer, and V. P. Shastri, Biomaterials, 2004, 25 3743; V. Pacheco-Fowler, T. Gaonkar, P. C. Wyer, and S. Modak, J. Hosp. Infect., 2004, 57 170; L. Rubinson and G. B. Diette, J. Lab. Clin. Med., 2004, 143 5; E. B. H. Hume, et al., Biomaterials, 2004, 25 5023; J. K. Baveja, G. Li, R. E. Nordon, E. B. H. Hume, N. Kumar, M. D. P. Willcox, and L. A. Poole-Warren, Biomaterials, 2004, 25 5013; R. M. Donlan and J. W. Costerton, Clinical Microbiology Reviews, 2002, 15 167; J. Thomas, S. B. Choi, R. Fjeldheim, and P. Boudjouk, Biofouling, 2004, 20 227; and N. Hilal, L. Al-Khatib, B. P. Atkin, V. Kochkodan, and N. Potapchenko, Desalination, 2003, 158 65.
A wide range of antimicrobial agents have been applied to surfaces: antibiotics including chlorhexidine, rifampin and monocycline and others, silver/silver ions/silver compounds, hydantoin (also known as halamine) compounds, furanone compounds, and quaternary ammonium or phosphonium polymers. There have been a smaller number of non-permanently cationic antimicrobial polymeric materials prepared for use on surfaces, generally incorporating benzoic acid derivatives. In addition to the references cited above, see A. Tzoris, E. A. H. Hall, G. A. J. Besselink, and P. Bergveld, Analytical Letters, 2003, 36 1781; B. D. Kalyon and U. Olgun, Am. J. Infect. Control, 2001, 29 124; H. Q. Jiang, S. Manolache, A. C. L. Wong, and F. S. Denes, Journal of Applied Polymer Science, 2004, 93 1411; Y. Chen, S. D. Worley, T. S. Huang, J. Weese, J. Kim, C. I. Wei, and J. F. Williams, Journal of Applied Polymer Science, 2004, 92 363; Y. Sun and G. Sun, Macromolecules, 2002, 35 8909; K. Anguige, J. R. King, J. P. Ward, and P. Williams, Math. Biosci., 2004, 192 39; J. Lin, J. C. Tiller, S. B. Lee, K. Lewis, and A. M. Klibanov, Biotechnology Letters, 2002, 24 801; J. Lin, S. Qiu, K. Lewis, and A. M. Klibanov, Biotechnology Progress, 2002, 18 1082; J. Lin, S. K. Murthy, B. D. Olsen, K. K. Gleason, and A. M. Klibanov, Biotechnology Letters, 2003, 25 1661; J. C. Tiller, S. B. Lee, K. Lewis, and A. M. Klibanov, Biotechnology and Bioengineering, 2002, 79 465; J. C. Tiller, C.-J. Liao, K. Lewis, and A. M. Klibanov, Proceeding of the National Academy of Sciences, 2001, 98 5981; S. B. Lee, R. R. Koepsel, S. W. Morley, K. Matyjaszewski, Y. J. Sun, and A. J. Russell, Biomacromolecules, 2004, 5 877; Y. A. G. Mahmoud and M. M. Aly, Mycopathologia, 2004, 157 145; H. Irikura, Y. Hasegawa, and Y. Takahashi, J. Photopolym Sci. Technol., 2003, 16 273; E. S. Park, H. K. Kim, J. H. Shim, M. N. Kim, and J. S. Yoon, Journal of Applied Polymer Science, 2004, 93 765; and R. Chuanchuen, K. Beinlich, T. T. Hoang, A. Becher, R. R. Karkhoff-Schweizer, and H. P. Schweizer, Antimicrob. Agents Chemother., 2001, 45 428.
The various agents are most often physically applied to the surface, physically impregnated into the bulk of the material, or physically incorporated into a coating that is then applied to the surface for “controlled release”. In all these approaches the antimicrobial agent leaches from the surface, leading to two key problems: a limited time of effectiveness; and environmental, health and safety concerns, such as the promotion of drug resistant microbes. Non-leaching antimicrobial surfaces have been created by covalently grafting an antimicrobial polymer to the surface, atom transfer radical polymerization of an antimicrobial polymer directly from an initiating surface, and covalent attachment of an agent to a polymer chain. In the later case, any attachment scheme must not obscure the active moiety of the molecule. Also, particular care must be taken to ensure that the agent is actually covalently bound and is not just physically incorporated and that it is not releasing from the surface, which leads to the same issues discussed above for leaching antimicrobial agents.
Antibiotics have generally been employed for medical applications. Central venous catheters have been both impregnated with chlorhexidine and a silver compound and coated with rifampin/minocycline on the exterior and intraluminal surfaces to reduce successfully the rate of catheter-related blood stream infections. Each case uses two active agents in an attempt to reduce the promotion of resistant bacteria. These approaches have been successfully commercialized and are now recommended for use in certain situations, and have lead to significant reductions in mortality and healthcare costs in some hospitals. H. A. Hanna, Raad, II, B. Hackett, S. K. Wallace, K. J. Price, D. E. Coyle, and C. L. Parmley, Chest, 2003, 124 1030. Aside from increased cost, other factors have slowed widespread adoption of commercially available catheters that have been impregnated or coated with antibiotics. These include concerns about the emergence of drug resistant bacteria, (although this is still under study for the specific case of impregnated catheters) and cases of anaphylactic shock reaction to chlorhexidine impregnated catheters have been reported. S. B. Levy and B. Marshall, Nat. Med., 2004, 10 S122; and R. Stephens, M. Mythen, P. Kallis, D. W. L. Davies, W. Egner, and A. Rickards, Br. J. Anaesth., 2001, 87 306. Some in the medical profession are uneasy about employing a leaching strategy in medical devices wherein active agents are released into a compromised patient. In such cases, native and beneficial bacteria populations (e.g., E. coli in the intestines) may be reduced, allowing pathogenic species to gain a foothold in the patient, among other side effects. In addition to central venous catheters, the use of antibiotics has also been explored in various devices, such as a coating on wires and pins, impregnated in endotracheal tubes, and slow release from periodontal implants. In addition to medical devices, antibiotics have been covalently bound to a polymer backbone for use in a biosensor and water systems. In theory, covalently bound antibiotics would never be released, and so should not promote resistant bacteria. However, it is not yet clear what effect, if any, covalently bound antibiotics may have on the promotion of drug resistant bacteria.
Silver, silver ions, and silver compounds have been used for a somewhat more varied range of applications. Medical devices impregnated with both an antibiotic and a silver compound were discussed above. In addition, urinary catheters with a silver alloy/hydrogel coating have also been examined. Various vapor deposition methods have been employed to coat fabric and polymer/metal surfaces. No matter how the silver component is incorporated it can only work as a leaching agent because it only kills the cells after being taken up by the bacterium. Hence, any system utilizing silver will have diminishing effectiveness over time. The length of effectiveness can be increased by incorporating more silver, but at some point this becomes untenable. In addition, patient sensitivity to silver compounds and coatings has been reported. C. K. Chan, F. Jarrett, and J. A. Moylan, J. Trauma-Injury Infect. Crit. Care, 1976, 16 395; J. Viala, L. Simon, C. Le Pommelet, L. Philippon, D. Devictor, and G. Huault, Arch. Pediatr., 1997, 4 1103; and P. Tozzi, A. Al-Darweesh, P. Vogt, and F. Stumpe, Eur. J Cardio-Thorac. Surg., 2001, 19 729. In one case the patient showed no allergic reaction to topical (skin) application of silver ions yet had a strong adverse reaction to internal use of a device coated with silver. As previously discussed, the polymers system here are non-leaching and so will not lose effectiveness over time. Patient sensitivity to the new polymers has not yet been studied, but presumably a polymer system can be found that has minimal sensitivity/allergy issues.
Various hydantoin, also known as halamine, compounds have been successfully incorporated as polymer pendant groups or grafted to fabrics to impart antimicrobial action. Sun et al. have created a variety of hydantoin moieties and both incorporated them into polymer beads for water purification applications and grafted them onto various textiles to provide enhanced protection against bacteria. Worley et al. also created polymer beads with hydantoin pendant groups for water purification for comparison to polymer beads with quaternary ammonium pendant groups and found the hydantoin beads to be more effective. The hydantoin moieties are essentially storage compounds for chlorine, which is released to the impinging bacterium to kill it. Therefore, while not technically a leaching material, eventually the material is exhausted of antimicrobial protection and must be “recharged.” Often, this can be done by rinsing the fabric in a sodium hypochlorite solution. However, this makes the material undesirable for cases where long term protection is desired and recharging is not realistic. In addition, the amine-halogen bond is photosensitive, somewhat limiting the use of these materials.
Furanones have been incorporated into a polymer matrix and covalently bound to the surface of catheters. Furanone compounds stop the growth of biofilms, a major route to bacterial toxicity attributed to biomaterials, by interrupting cell-to-cell communication. They do not kill individual bacterium; instead the agent simply stops them from communicating as a population to form a biofilm on a surface, and so no attachment to the surface takes place. Because of this mode of action, the authors propose that furanones are unlikely to induce bacterial resistance. However, bacteria have been able to develop resistance to a wide variety of antibiotics that act on one particular metabolic pathway. While it has not yet been shown that bacteria can develop resistance to furanones, it is easy to propose a theoretical mechanism wherein bacteria evolve to overcome the action of these agents. In addition, it is unclear that simply stopping the formation of a biofilm on a device but allowing bacteria to live will reduce infection rates. Indeed, despite reduced bacterial adhesion to the coated catheters, only a slightly reduced level of infection at the implant site in an animal model trial is reported.
Numerous quaternary ammonium, and to a lesser extent phosphonium, compounds and polymers have been shown to be effective antimicrobial agents. This work will mainly be concerned with quaternary ammonium polymers used on solid surfaces. Klibanov et al. covalently bound quaternary amine polymers, for instance poly(4-vinyl-N-alkylpyridinium bromide) and alkylated poly(ethyleneimine), to nonporous substrates and textiles by various methods for a range of potential applications. Polyquaternary amines have also been grafted to water filtration membranes for use in biofouling applications grown by atom transfer radical polymerization from a fabric surface and condensation of siloxyl compounds with an attached quaternary amine moiety. Several other quaternary ammonium and phosphonium polymers are described in a recent review. All these polymers are permanently cationic. It is thought the mechanism of action is association with the slightly negatively charged cell membrane followed by penetration and disruption of the membrane releasing the cell contents. They were shown effective against both Gram positive and Gram negative bacteria, including bacteria resistant to cationic small molecule drugs such as MRSA, and fungi. The covalent attachment schemes varied, but all involved immersing the substrate in various solvents for long periods of time and/or elevated temperatures. Often, the grafting surface required a specific functional group to covalently bond the polymer. A minimum of two steps were required in each case, grafting the polymer to the surface followed by quaternization of the amino functional group. It is possible to polymerize the quaternized monomer in some cases. The polymer coating described herein is applied to any substrate by initiated chemical vapor deposition. This process is completely solventless and the substrate is maintained at near-room temperature; thus substrates that are solvent and/or heat sensitive can be easily coated. Also, the polymer is not quaternized to a permanently cationic state, reducing the number of processing steps and processing time and avoiding a second step requiring the use of harsh solvents. The polymer can be covalently bonded to a very wide range of substrates using a grafting procedure described in more detail below. The grafting procedure is also an all-vapor phase process, with no solvents and/or elevated temperatures required.
A few antimicrobial polymers that do not contain quaternary amine or phosphine moieties have been synthesized. These have incorporated pendant groups of benzoic acid derivatives attached to a polymer backbone or benzoic acid in the backbone of a polyimide coating. In the former case the goal was to make a bulk antimicrobial polymer for use in biomedical applications instead of a surface coating. The later case has the most relevance to this work as the polyimide coating was formed by a solventless vapor deposition process, as were the polymers described herein. However, the process developed by Irikura et al. requires that the substrate withstand high temperatures, about 200° C., and thus can essentially only be used on metal substrates.
In addition, Gellman et al. developed an antimicrobial polymer that is not permanently cationic. M. Gelman, B. Weisblum, D. Lynn, and S. Gellman, Organic Letters, 2004, 4 557. Instead, the amino moiety has a pKa of about 10, and so the nitrogen atom is protonated to a significant extent at physiological conditions, resulting in a cationic polymer. The polymer, poly(dimethylaminomethylstyrene) was designed to adopt a similar conformation in solution as that of antimicrobial peptides. Gellman et al. compared their polymer to the similar polymer in which the amino group was quaternized and they found the non-quaternized polymer was more effective. However, the antimicrobial testing was carried out in solution and it was not apparent the same would hold true for the polymer applied to a surface, particularly because presumably the polymer would not be free to adopt the favorable conformation.